Cycles fireflies with police lights setup
In the glass shader you need to use the light path node. Setup your material so the main section gets mixed with a full transparent shader by way of the camera output of the light path node.



This will make your glass look like glass when the camera sees it, but not even exist when other objects see it. Doing the same with the bulb and then adding a point light would also be an improvement to sample count.

Note: This is not physically correct. To get better render times from cycles, you need to pull tricks like this to improve sample count.

Edit:

After playing with your file I got this (25 samples):



Here's what I did - switched device to CPU and disabled reflective caustics. (I also did a little trick with smooth shading to improve the look of the siren, but that's another topic.)



Here's the new blend: